The invention relates to cutter assemblies for rotary drag-type drill bits, for use in drilling or coring holes in sub-surface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutter assemblies.
In drag-type drill bits of this kind the bit body may be machined from metal, usually steel, sockets to receive the cutter assemblies being drilled into the bit body. Alternatively, the bit body may be formed by a powder metallurgy process. In this process a hollow mould is formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered matrix-forming material, such as powdered tungsten carbide, which is then infiltrated with a metal alloy, such as a copper alloy, in a furnace so as to form a hard matrix. Using conventional infiltration alloys, the furnace temperature required to form the matrix is usually of the order of 1000.degree. C. to 1170.degree. C.
The present invention relates to the manufacture of cutter assemblies of the kind in which a preform polycrystalline diamond cutting element is mounted on a carrier of material which is less hard than the diamond, the carrier then in turn being secured within a socket in the bit body.
A common form of cutting element comprises a flat tablet, usually circular, having a front cutting table of polycrystalline diamond bonded to a substrate of less hard material, such as cemented tungsten carbide. The layer of polycrystalline diamond is formed and bonded to the substrate in a high pressure, high temperature press, and one or more transition layers may sometimes be provided between the cutting table and substrate. The general details of manufacture of such cutting elements are well known and do not form a part of the present invention.
The carrier is usually in the form of a cylindrical post or stud and may, for example, also be formed from cemented tungsten carbide. Each cutting element is normally mounted on its carrier by brazing the rear surface of the substrate to a surface of the carrier. However, two-layer and multi-layer cutting elements of the kind described tend to degrade when subjected to very high temperatures, and in this case they are therefore often referred to as being non-thermally stable. As the temperature to which the cutting elements are subjected increases, differential expansion between the layers of the element may cause delamination or separation of the diamond layer from the substrate. Very high temperatures may also lead to degradation of the polycrystalline diamond material itself. In view of this, special brazing processes have to be used when brazing such a non-thermally stable cutting element to its carrier, to ensure that unacceptable degradation of the cutting element does not occur. One such brazing process is known as "LS bonding".
There also exist polycrystalline diamond cutting elements which are referred to as thermally stable. These normally consist of only a single body of polycrystalline diamond of a particular type, not bonded to a substrate.
Currently, conventional two-layer or multi-layer cutting elements are usually regarded as not being thermally stable above a temperature of about 750.degree. C. However, it will be appreciated that, for any given cutting element, there is not an exact critical temperature at which thermal degradation suddenly occurs, and it is possible that some "non-thermally stable" cutting elements might, in practice, be able to withstand temperatures somewhat in excess of 750.degree. C. For the purposes of this specification, therefore, "thermally stable" cutting elements will mean polycrystalline diamond cutting elements which can be subjected to some temperature in excess of about 1000.degree. C. without suffering significant thermal degradation, whereas cutting elements which would begin to suffer significant thermal degradation at a temperature which is less than about 1000.degree. C. will be referred to as "non-thermally stable".
An object of the invention is to provide a method of manufacturing cutter assemblies incorporating non-thermally stable cutting elements where the risk of thermal degradation of the cutting elements is reduced.